The present invention relates to a method and apparatus for controlling the cuff pressure in a non-invasive sphygmomanometer. The cuff pressure control mechanism comprises one or a plurality of valves whose operation ion is controlled by means of one or a plurality of control elements in a manner that the valve or valves have an operating frequency which is above the heart beat rate of the heart.
The blood pressure is generally tested by pumping air into a cuff which is wrapped e.g. around the arm of a patient. When the cuff pressure is sufficiently high to stop arterial blood circulation, the reduction of pressure is initiated. As the cuff pressure drops so-called Korotkoff sounds are listened for. As the sound becomes audible, the reading of systolic pressure is obtained. As the pressure drop is continued, the Korotkoff sounds gradually cease altogether. The reading of diastolic pressure is obtained at the moment the sounds cease to be audible.
The Finnish Patent application No. 853781, corresponding to U.S. Pat. No. 4,768,518, described an apparatus used in automatic blood pressure testing, comprising a cuff, a pump and at least two magnetic valves provided with a throttle element, as well as a microprocessor. The throttle element consists of a thin tube whose length has an effect on the pressure reduction rate. It is customary to employ a plurality of throttle elements in a single apparatus, each throttle element being fitted with a tube of different length. Each throttle element is controlled by its own magnetic valve. The magnetic valves are controlled by means of a microprocessor which opens a given magnetic valve depending on which magnetic valve has behind it a throttle element that is best suitable for maintaining a linear cuff air pressure reduction rate at a given moment. If necessary, several magnetic valves can also be opened at the same time. A problem in this type of apparatus is that it requires a plurality of bulky components consisting of a magnetic valve and a throttle element and, nevertheless, it is not possible to obtain a linear rate of the cuff pressure reduction. In addition, opening an of valves leads to disturbances in testing. Also, a pressure drop profile cannot be freely adjusted.
The cuff pressure reduction can also be controlled by using a stepping motor driven needle valve. In order to maintain a uniform pressure drop, the valve must be adjusted during testing. This type of system requires expensive equipment as the manufacturing process is quite complicated. Another problem in this apparatus is malfunctions caused by dust.